1. Field of Invention
This invention relates to a method and apparatus for automatic white shading correction of video signals with the correction derived from using a flat white calibration target.
2. Description of Related Art
In scanned detector systems such as television (video) cameras, problems arise when viewing a perfectly white, featureless, calibration target. Video signals resulting from viewing a perfectly white calibration target with a current technology raster scan camera do not correspond to a "flat white" video signal at the output, but instead contain various amounts of undesired distortions or shading. Different factors cause these distortions, among them the inherent non-uniformity in the sensitivity of the video camera's photo electric devices and its optical system imperfections. The undesired shading can be considered as a "calibration" error in the analog output video signal that results in inaccurate reproduction of video signal amplitudes in other scenes to be imaged with the same video camera. White shading adjustment corrects such inherent "calibration" errors. Historically such errors were compensated for by manipulating the gain of the video amplifier on a line-by-line or pixel-by-pixel basis.
As an example, in video cameras, various adjustable analog signals such as sawtooths and parabolas applied in both horizontal and vertical directions can be used to modulate the output video signals in an attempt to correct the analog video signal while calibrating the camera against a flat white calibration target. Typically, such white shading correction is performed manually by viewing the video output waveform from the camera on a monitor and then manually introducing varying amounts of horizontal and vertical corrections to minimize or eliminate the white shading distortions. The manually varied sawtooth or parabola waveforms are used to compensate the video signal from the camera to produce a less distorted output more closely resembling the imaged flat white calibration target. Inherent in these prior art systems is the requirement that an operator monitor the video waveform while a calibration target is being imaged and then manually input the corrections to correct for white shading distortions.
However, this analog waveform method of white shading correction does not conveniently allow for the introduction of higher order corrective waveforms for two reasons. First, the information necessary to properly determine these higher order waveforms cannot be efficiently derived from visual inspection. Second, even if such higher order corrective analog waveforms would be provided, it is difficult to manually set them to yield optimal corrections.
Another limitation, in the prior art is that only a limited number of correction points are available as compared to the total number of pixels (image elements or data) in an image, therefore the corrections that can be derived from this limited number of points is by necessity incapable of correcting for errors detailed to the level of individual pixels. Furthermore, typically the waveform needed to perform the correction function is available separately from where it needs to be applied. This lack of coordination between the identification of the corrective waveform, and its application at the pixel level limits the usefulness of the prior art systems.
In view of the limitations of the prior art, it is an object of the present invention to provide a method and apparatus which enables automatic white shading correction of a video signal where the determination of the corrective waveform as well as its application to the signal to be corrected is achieved within the same apparatus.
It is another object of the invention to provide a method and apparatus for detecting a signal needing white shading correction and, as a result of this detection, performing the required white shading correction.
It is still another object of the invention to provide a method and apparatus for detecting a signal needing white shading correction by evaluating each pixel of a video signal and calculating white level correction coefficients for each pixel to be used subsequently during the acquisition of other video images.
It is yet another object of the invention to provide a method and apparatus for reducing the influence of noise during the determination of a white shading correction.